1. Field of the Invention
This invention relates to a method and system for evaluating characteristics of an S-cam used in drum brakes and, in particular, to a method and system that improve the ability to measure deviation of a profile of the cam from a desired profile.
2. Discussion of Related Art
In a conventional drum brake, a brake drum rotates with a wheel or wheels proximate one end of an axle. The brake drum defines a radially inner braking surface. A brake spider is disposed about the axle and supports an anchor pin that extends axially from the spider. A pair of brake shoes are pivotally mounted on the anchor at a first end. Each brake shoe includes a roller, or cam follower, proximate a second end. A generally S-shaped cam engages the rollers in the brake shoes and rotation of the cam moves the brake shoes between positions of engagement and disengagement with the braking surface of the brake drum.
Proper dimensioning of the S-cam is critical to consistent application of the brake shoes against the braking surface of the brake drum. Deviation of the profile of either lobe of the S-cam from an optimal profile can lead to uneven application of braking force and uneven wear of braking components. Deviation among the profiles for the two lobes can also result in uneven application of braking force among the two brake shoes. S-cams are therefore often inspected during the manufacturing process to determine the extent of deviation from a desired profile.
Conventional methods and systems for inspecting S-cams suffer from several deficiencies. Conventional methods and systems used to inspect S-cams often rely on manual movement and measurements. As a result, the methods and systems are time consuming and relatively inaccurate. Further, conventional methods and systems fail to account for one or more important characteristics of the S-cam identified by the inventors that contribute to effective application of the S-cam. Coordinate measuring machines (CMMs) are used in many industries to measure surface profiles of various objects (including, for example engine camshafts) and overcome the inefficiencies and inaccuracies inherent in some of the manual measurements but are still highly dependent on the operator. CMMs are typically general purpose devices and/or primarily directed to parts other than the S-cam, however, and fail to account for the various characteristics of the S-cam identified by the inventors that contribute to effective application of the S-cam.
The inventors herein have recognized a need for a method and system for evaluating characteristics of S-cams that will minimize and/or eliminate one or more of the above-identified deficiencies.